1. Field of the Invention
This invention relates to a device and method for transmitting seismic signals into the earth from a surface or subsurface location, in which the signal is produced by the vibration of a piezoelectric transducer. In one embodiment, the device can be operated with a borehole. In another embodiment, the device can be permanently buried at a subsurface location.
2. Discussion of the Art
Information about subsurface geological characteristics is routinely obtained by oil and gas exploration companies using seismic technology. A seismic energy source generates sound waves which are reflected and refracted as they pass through the earth. The signal is recorded at another location and analyzed.
Conventional methods for generating a seismic signal include explosives and vibrators. For example, detonation of dynamite in a hole can produce a single shock vibration. Truck-mounted heavy vibrators such as VIBROSEIS (trademark of Conoco) units are capable of multiple vibrations at controlled frequencies. For offshore exploration, gas exploders or other devices can be towed behind a boat.
Additional methods are available but are not in widespread commercial use. For example, U.S. Pat. No. 3,298,456 to Picou discloses a seismic wave generator in which the driving force is provided by a magnetic coil.
In addition to devices used on the earth's surface, downhole periodic seismic sources, such as a pneumatic oscillator discussed by H. C. Hardee ("Downhole Periodic Seismic Sources, "Geophysical Prospecting, 31: 57-71, 1983), have been proposed. Advantages include higher signal efficiency, improved resolution, and the capability for repetitive study of deep structures. Downhole periodic sources can be used with downhole geophones for improved vertical seismic profiling, or they can be used with surface receivers.
Existing seismic signal generators have not been completely satisfactory. Inefficiencies in hydraulic or pneumatic vibrators result in wasted energy and diminished signal resolution as signal frequency increases. There is a need for a signal generator which is easily controlled and which avoids the shortcomings of existing vibrators.
Piezoelectric transducers are not new to seismic prospecting; they are sometimes used in receivers for seismic signals known as accelerometers. Surface accelerometers are typically laid out in a spaced pattern some distance from the seismic source and have a short spike which is pushed into the soil to provide a firm coupling with the earth's surface. Vibration of the accelerometer causes vibration in a piezoelectric crystal, which generates a voltage in relation to the received signal.
Hydrophones, sensors adapted for receiving seismic signals through water, often incorporate piezoelectric transducers. Other examples of piezoelectric crystals converting mechanical energy to electrical energy are phonograph cartridges and microphones.
Piezoelectric crystals can also convert electrical energy to mechanical energy. Examples of such applications include sonar, fluid flow measurement, and ultrasonic jewelry cleaning baths. They have been used for dynamic positioning of observatory telescope mirrors and other large items.
The patent literature discloses other uses for piezoelectric crystals. U.S. Pat. No. 4,491,759 to Kunz et al, describes a piezoelectric vibration exciter for generating mechanical vibrations over a 20 kHz to 100 kHz frequency range for the purpose of non-destructive material testing. The piezoceramic disk stacks are mechanically connected to a test specimen, and the disks vibrate at the frequency of an applied AC voltage.
Ultrasonic piezoelectric transducers, designed to generate acoustical wave trains having frequencies of 50 kilocycles or more, and preferably greater than 100 kilocycles, are disclosed as useful in well logging in U.S. Pat. No. 3,949,352 to Vogel. A beam of ultrasonic energy is transmitted from the borehole to produce shear waves in the adjacent formation, and the return signal is monitored from another location in the same borehole.
U.S. Pat. No. 4,245,172 to Shirley describes an improved transducer using a plurality of piezoelectric elements in a bimorph configuration for generating and detecting shear waves for the analysis of solid and semi-solid materials. The resulting displacement from this configuration is relatively high but the force is weak compared to other piezoelectric configurations.
It is an object of this invention to provide a more efficient seismic energy source by converting electrical energy directly into acoustic energy and to avoid energy disadvantages associated with hydraulic and pneumatic systems, such as heat loss through friction, viscosity, and turbulence.
It is another object of this invention to conserve energy compared to seismic methods involving dropped or vibrating weights. Most of the energy expended in this invention by raising the weight is stored and reclaimed in the subsequent downward cycle, while in conventional hydraulic VIBROSEIS technology much energy is wasted as heat.
It is a further object of this invention to provide an efficient vibratory source signal that does not degrade at higher frequencies (e.g., above 100 Hz).
It is an additional object of this invention to produce a vibratory seismic device with dimensions and capabilities that would make it suitable for temporary or permanent placement in a borehole.